This invention relates to an improved receiver selecting system and, particularly, to a system for selecting from a plurality of receivers the one receiver which provides the highest quality signal.
In mobile communication systems, the talk back range of the mobile or portable transceiver is limited because of the output power capabilities. The talk back range can be extended by placing additional receivers in strategic locations, so that during transmission from a portable or mobile unit from any location in the expected coverage area, at least one receiver will receive a signal of sufficient strength to effectively communicate to the dispatcher or main station. In the majority of cases, an acceptable signal will be present at more than one of these satellite receivers.
The audio from each satellite receiver is generally transmitted over telephone lines to a central location where electronic hardware is used to determine which receiver has the best quality audio which is then fed to a common output. The system by which the selection of the best quality signal from a number of inputs is provided, as described above, is referred to herein as the receiver selecting system. This type of system is sometimes known as "receiver voting." To further complicate matters, the signal to noise (S/N) ratio from a particular satellite receiver can change dramatically from the best choice to one that is un-acceptable during a signal transmission due to "Rayleigh fading." The fading rate is determined by the channel frequency and the relative motion between the transmitting and receiving antennas. In the 900 MHz band, the fading rate can approach 10 milliseconds.
Systems or arrangements for selecting the best signal from a plurality of radio receivers are known in the prior art, as exemplified by U.S. Pat. Nos. 3,403,341; 3,495,175 and 3,729,681. The prior art systems have various disadvantages, some of which are: a selecting arrangement which makes irrevocable selection at the beginning of a radio transmission, even though the selected receiver may subsequently provide a poor signal; a selecting arrangement which uses signals indicated by audible tones which are coded to indicate the best signal, but which must be filtered to avoid interference; a selecting arrangement which is relatively slow in operation so that some signals may be lost; a selecting arrangement in which a weak signal with excellent signal-over-noise is excluded in favor of a strong signal with poor quality; and a selecting arrangement which, although giving good indications of the best received signal, is relatively complex.